1. Field of the Invention
The present invention relates to an image forming apparatus having a thermal fixing unit, and more particularly, to an image forming apparatus which detects the temperature of a thermal fixing unit to control the driving system of the image forming apparatus.
2. Description of Related Art
There are various types of image forming apparatus known which include thermal fixing units. As an example, a photosensitive pressure-sensitive copying machine as shown in FIG. 4 will be described. Since the controller of the copying machine illustrated in FIG. 4 has substantially the same structure, except for controller 31 and sensor 30, as the one disclosed in U.S. patent application Ser. No. 156,744 (filed Feb. 18, 1988), now U.S. Pat. No. 4,893,147, assigned to the same assignee of the present application, its detailed description will be omitted.
The photosensitive pressure-sensitive copying machine 1 uses a photosensitive sheet comprising a photosensitive pressure-sensitive sheet (microcapsule sheet) 11 and a developer sheet 16. Disposed at the top portion of the copying machine 1 are an original support stand glass 2 and a liftable stand cover 3. An original 4 is placed face down on the original support stand glass 2. Light emitted from a halogen lamp 5 is reflected by the original 4 and reaches an exposure stand 10 along the path indicated by the one-dot chain line in FIG. 4.
A detachable cartridge C contains an elongated microcapsule sheet 11. The sheet 11 fed out from the cartridge C travels under the exposure stand 10 and passes through a pressure developing unit 13 before being wound around a take-up shaft 15. The microcapsule sheet 11, when passing under the exposure stand 10, has a latent image formed thereon by the light reflected from the original 4.
A developer sheet 16 is pressed by the pressure developing unit 13 while it is associated with the portion of the microcapsule sheet 11 on which the latent image is formed. The pressure application ruptures light-unexposed or non-photocured microcapsules on sheet 11 and forms an image on the developer sheet 16. The developer sheet 16 is then delivered to a thermal fixing unit 20 where the image is thermally fixed by a heat roller 20a. Conventionally, an image forming apparatus with a thermal fixing unit, as shown in FIG. 4, detects the temperature of the thermal fixing unit to control the main driving system.
The following will describe a case where a prior art thermal fixing unit is applied to the copying machine shown in FIG. 4. A temperature sensor 30 will be provided near the thermal fixing unit 20 to detect the ambient temperature. An output signal from this sensor 30 is inputted to a controller 31. When the temperature sensor 30 detects that the temperature near the thermal fixing unit is low due to the heat roller 20a not being properly driven when the aforementioned main driving system is functioning, the controller 31 stops a main driving system to prevent a developer sheet from being discharged. This prevents a developer sheet from not being properly subjected to a thermal fixing process.
When the supply of microcapsule sheet 11 is exhausted, the cartridge C is removed from the copying machine 1 and is replaced with a new one. Since the thermal fixing unit 20 includes the heat roller 20a, the temperature of the old cartridge C is slightly increased. Immediately after a new cartridge C, which has been generally stored in a cool and dark place, is loaded, the temperature inside the image forming apparatus rapidly decreases. Accordingly, at the time the cartridge replacement takes place, the rapid decrease in temperature in the image forming apparatus causes the temperature sensor 30 to output a low temperature signal to the controller 31 even if the heat roller 20a has been activated and is functioning properly. Upon reception of this low temperature signal, the controller 31 stops the main driving system. This results in undesired interruption or stopping of various operations, such as auto sheet feeding from the new cartridge and the copying operation, which are executed under the normal conditions, until the temperature detected by sensor 30 increases to the desired level. Interruption of the auto sheet feed operation is inconvenient and causes unnecessary delays in the time required for replacing a used microcapsule cartridge with a fresh cartridge. After placing a fresh cartridge in the image forming machine, the leading end of the fresh role of recording medium must be fed through a series of rollers and wound around take-up shaft 15. If the controller is stopping the main driving system due to reception of the low temperature signal, the auto sheet feeding operation cannot begin. Since the low temperature signal will continue until the fresh cassette is heated, the time period required for replacing recording medium cartridges in the previously known image forming apparatus includes the time required for the fresh cassette to be adequately heated. The time required for the fresh cassette to be heated results in an undesirable delay in the exchange time.